1. Field of the Invention
The invention relates to a field-emission display device. It is applicable to display screens of the flat-screen type and in particular high-resolution (100 xcexcm pixel spacing), high-luminance (up to 500 cd/M2) and low-consumption screens. It is also applicable to the production of a planar microgun electron source applicable especially in microlithography.
2. Discussion of the Background
A field-emission display (FED) screen is schematically composed of a cathode, an anode and an interelectrode space under vacuum. The cathode is a matrix of electron emitters which illuminate the anode where various phosphors, that is to say receptors, are placed. Since corresponding to each emitter there is a receptor, the resolution of a direct-viewing screen is defined by the interpixel spacing with which it is manufactured.
For small (less than 14 inch diagonal) high-resolution screens, this spacing is about 100 to 300 xcexcm by 100 to 300 xcexcm. Direct-viewing screens having the highest resolution are without doubt avionic screens which have to be manufactured with a pixel pitch of about 100 xcexcm by 100 xcexcm. In colour displays, the dot pitch is greater since a dot is composed of three-red, green and blue-pixels.
In order to avoid the phenomenon of colour crosstalk, 99% of the electrons emitted by an emitter must strike the receptor which corresponds to it. The size (fT by fT) of the beam, emitted by an emitter of size fE by fE, at the anode is equal to: fT(xcexcm)=fE+2X, 2X being the broadening of the beam with respect to its initial size. For example, for a 40 by 40 xcexcm emitter size, X must be less than or equal to 30 xcexcm.
If each element emits a beam of electrons having an initial velocity vi in a cone of half-angle q, the anode-cathode distance dca may be written in the form of the following formula:       d    ca    =                    qE                  2          ⁢          m                    ·              t        2              +                  v        0            ⁢      t      
with E: cathode-anode field (v/m)
m: electron mass: 9.1xc3x9710xe2x88x9231 kg
q: electron charge: 1.6xc3x9710xe2x88x9219 C
t: cathode-anode transit time (s)
v0: orthogonal component of vi (m/s).
Since xc2xdmvi2=qEi and v0=vi cos xcex8,
where qEi is the initial energy of the electrons (eV), then:             t      2        +                                        8            ⁢                          mE              i                        ⁢                          cos              2                        ⁢            θ                                qE            2                              ⁢              xe2x80x83            ⁢      t        -                            2          ⁢          m                qE            ⁢              xe2x80x83            ⁢              d        ca              =  0
The solution of this equation is:   t  =                                          2            ⁢                          mE              i                        ⁢                          cos              2                        ⁢            θ                                qE            2                          +                              2            ⁢                          md              ca                                qE                      -                            2          ⁢                      mE            i                    ⁢                      cos            2                    ⁢          θ                          qE          2                    
Since       X    =                            v          p                ⁢        t            =                                                  2              ⁢                              qE                i                                      m                          ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢                  θ          ·          t                      ,
where vp is the parallel component of vi (m/s), then:                     X        =                  2          ⁢                      xe2x80x83                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ            ⁡                          (                                                                                                                                            E                          i                          2                                                ⁢                                                  cos                          2                                                ⁢                        θ                                                                    E                        2                                                              +                                                                  d                        ca                                            ⁢                                              xe2x80x83                                            ⁢                                                                        E                          i                                                E                                                                                            -                                                                                                    E                        i                        2                                            ⁢                                              cos                        2                                            ⁢                      θ                                                              E                      2                                                                                  )                                                              X        =                  2          ⁢                                                    E                i                            E                                ⁢                      xe2x80x83                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ            ⁡                          (                                                                                          d                      ca                                        +                                                                                            E                          i                                                E                                            ⁢                                              xe2x80x83                                            ⁢                                              cos                        2                                            ⁢                      θ                                                                      -                                                                                                    E                        i                                            E                                        ⁢                                          xe2x80x83                                        ⁢                                          cos                      2                                        ⁢                    θ                                                              )                                          
In general (see examples described below), in order to avoid cathode-anode breakdown phenomena, dca is chosen to be equal to dca(mm)=xc2xd Va(kV), which corresponds to a field E=2xc3x97106 V/m.
It should be noted that for low-energy (≈1 eV) electrons, the term (Ei/E)cos2xcex8 becomes negligible. This is because (Ei/E)cos2xcex8xe2x89xa6Ei/Exe2x89xa65xc3x97107 m less than  less than dca.
The constraint on the luminance (500 cd/M2) corresponds to a luminosity of 1600 Lm/m2 and therefore to 1.6xc3x9710xe2x88x925 Lm per pixel (100 by 100 xcexcm pixel). Taking a phosphor efficiency of 5 Lm/W (for electrons having an energy of 5 keV), we obtain 3.2 xcexcW per pixel, which corresponds to an average current of 0.64 nA. Since each pixel emits during the time that the corresponding line is being addressed, the emission current per pixel must be 0.64 xcexcA (for a screen with 1000 lines). This pixel current corresponds to current densities of 10 mA/cm2, 18 mA/cm2 and 40 mA/cm2 for 80 by 80 xcexcm, 60 by 60 xcexcm and 40 by 40 xcexcm emissive sources, respectively.
In order to determine a quality criterion for a screen with respect to the power dissipated for its operation, it is possible to define a parameter characteristic of the power needed to go from a black pixel to a white pixel, namely:   P  =            1      2        ⁢          xe2x80x83        ⁢                            C          p                ⁢                  V          scan          2                            t        c            
where Cp is the capacitance of a pixel, Vscan is the difference between the addressing voltage for a white pixel and for a black pixel and tc is the charging time of the pixel, which is of the order of 10 xcexcs. Consequently:
P(xcexcW)=0.05 xc3x97Cp(pF)xc3x97Vscan2.
It should be noted that in the case of a liquid-crystal screen (Cp≈0.6 pF and Vscan=10 V), this parameter P is equal to 3 xcexcW.
Within the technology of field-effect screens, the screen manufactured by the company Pixtech [1] is known. This screen uses a cathode with field-emission tips. Each emitter is composed of about 30 tips or more. According to S. T. Purcell et al. [2], the beam emitted by this type of cathode is composed of primary electrons having an initial energy of about 10 eV less than the gate voltage and of secondary electrons having an average energy of 7 eV. Assuming electrons with an initial energy of 90 eV (gate voltage=100 V) emitted in a cone of about 30xc2x0 half-angle and striking an anode biased at 400 V, a distance dca equal to 0.2 mm and X=69 xcexcm are obtained. Since the emitting surface seems to be about 40 xcexcm along the axis for which the pixel pitch is 100 xcexcm, a beam size of the order of 180 xcexcm is obtained. According to Futaba [1], xcfx86T is equal to 230 xcexcm for 95% of the electrons emitted by an emitter. In order to obtain a beam size of less than 100 xcexcm, Futaba and Pixtech use the switched-anode technique: dual anode [1] and triple anode [3]. In these configurations, a switched anode is flanked by non-selected and therefore non-biased, anodes. As a result, the electrons are focused onto the selected anode. The size of the beam at the anode is then less than 100 xcexcm. However, since the distance between anodes is of the order of 30 xcexcm, it would seem to be impossible to use a high anode voltage (greater than 1 kV). Since low-voltage phosphors have a low efficiency, the present results are not very satisfactory since the luminance of the screen obtained is low: 80 cd/m2 instead of 500 cd/m2 for an avionics screen.
Since the capacitance of a pixel is given by:
Cp=xcex50.xcex5rS.1/e=0.009 pF
where e is the thickness of silica between the gate and the base of the tip: 1 xcexcm
xcex5r (silica): 4
S is the coverage area per pixel: 50 by 50 xcexcm.
The value of P(xcexcW) obtained is 0.05xc3x97Cp(pF)xc3x97V2scan=4 xcexcW with Vscan=30 V i.e. a value equivalent to that obtained for a liquid-crystal screen.
In order to obtain a high-resolution luminous screen, it is necessary to have a screen operating with an anode voltage ranging from 4 kV to 6 kV, for which the parameter X is small (≈30 xcexcm). To do this, the beam emitted by the cathode must have a low divergence and a low energy.
Materials with a low electron affinity are known, such as carbon with a diamond structure. This is a low-field emissive material, for example for a field of between 1 and 50 V/xcexcm, the emissivity of which is commonly ascribed to the low electron affinity of the material but which may be due to other phenomena. In the rest of the description, this material will be called xe2x80x9cmaterial with a low electron affinityxe2x80x9d as is done in the art. These materials have the great advantage of emitting electrons for low extraction fields (of the order of 10 V/xcexcm). Since it is easy to obtain such fields over a plane thin layer, it is no longer necessary to produce tips, thereby facilitating the fabrication process. For example, in a cathode with tips, it is absolutely essential to control the diameter of the holes in the extraction gate to within 0.1 xcexcm [7].
W. Zhu et al. [8] have studied deposited films of polycrystalline diamond obtained by CVD (chemical vapour deposition) and have shown that the emission density increased strongly with the density of defects that the films contain. Certain deposition conditions make it possible to obtain layers having, for fields of the order of 30 V/xcexcm, current densities of 10 mA/cm2, i.e. a value high enough to fabricate a screen with a luminance of 300 cd/m2. However, the emissive properties of the films do not seem to be very uniform since they depend greatly on the roughness (of the order of the grain size ≈5 xcexcm) and on the defect density [9].
The invention therefore relates to a structure of a field-emission device operating at low voltage, the cathode of which has a good surface finish.
The invention therefore relates to a field-emission device comprising at least one cathode made of material with a low electron affinity, characterized in that the material with a low electron affinity is an amorphous or crystalline material.